The present invention relates to a coil assembly for producing a gradient magnetic field useful in nuclear magnetic resonance (NMR) imaging and spectroscopy and, more specifically, to an assembly of at least one essentially planar gradient coil, each for providing a magnetic field gradient having a reduced rise time and increased gradient strength, to especially facilitate high-speed, high-resolution NMR imaging.
In NMR imaging systems currently employed for forming images of internal human anatomy, a patient is placed in a static magnetic field and is subjected to radio-frequency (RF) electromagnetic pulses. The nuclear resonances of the atomic nuclei of the patient are detected with an antenna, to provide information from which an image can be formed of that portion of the patient containing those excited nuclei. If the magnetic field possesses monotonic gradients in each of the three spatial dimensions, the position of the resonating nuclei can be determined. Magnetic resonance spectroscopy, for analyzing properties of the nuclei, utilizes the same phenomena. Typically, a main magnet (generally of superconducting nature) establishes a uniform high-strength magnetic field, upon which linear gradients are superimposed by gradient coils located within the bore of the main magnet. Generally, the gradient coils are comprised of windings etched upon a flexible printed circuit board, which is then curved to form a cylindrical coil. Highly linear coils, for achieving high resolution imaging, can be designed and fabricated as taught in U.S. Pat. No. 4,646,024, issued Feb. 24, 1987, and can be of a self-shielded nature, as taught in U.S. Pat. No. 4,737,716, issued Apr. 12, 1988, for eliminating eddy currents; both of these patents are assigned to the assignee of the present invention, and are incorporated herein in their entirety by reference. In certain applications, such as high-speed NMR imaging of the human heart, utilizing a "blip echo planar" technique, such as described in U.S. Pat. No. 4,451,788, issued May 29, 1984, assigned to the assignee of the present invention and incorporated herein by reference, images can be acquired in about 1/40-th of a second, if magnetic field gradients can be achieved with strengths of about 2 to 3 Gauss/cm. and rise times of about 100 microseconds. For a given gradient strength and rise time, the peak gradient coil power is proportional to the fifth power of the coil size. However, while substantial performance improvements can be realized by reducing the size of the cylindrical coil, with no change in gradient amplifier/coil subsystem power supplies, it will be apparent that the volume within the smaller gradient coil will rapidly decrease, so that high-speed, high-resolution NMR imaging of any appreciable volume becomes extremely difficult. Even if a useful volume is available, the total amount of data collected in a high-speed image is usually less than that for an image acquired over a much longer time interval, typically in the tens of minutes; high-speed images are, thus, generally of lower resolution and/or exhibit poorer signal-to-noise ratio, with respect to a convention NMR image. As a result, high-speed/high resolution NMR imaging presently has more limited applications than normal NMR imaging. It is therefore highly desirable to provide improved gradient magnetic field-forming means which allow the necessary response data for forming an entire image to be obtained in a relatively short time, and (ideally) without substantially modifying the gradient power amplifiers/supplies of a presently-existing typical NMR imaging system.